Type II porous ionic liquid based on metal-organic cage that enables L-tryptophan identification

Abstract

Porous liquids with chemical separation properties are quite well-studied in general, but there is only a handful of reports in the context of identification and separation of non-gaseous molecules. Herein, we report the first example of Type-II porous ionic liquid that exhibits exceptional selectivity towards L-tryptophan (L-Trp) over other aromatic amino acids and the first in which coordination cages promote this selectivity. A previously known class of anionic organic-inorganic hybrid doughnut-like cage (HD) has been readily dissolved in trihexyltetradecylphosphonium chloride (THTP_Cl). The resulting liquid, HD/THTP_Cl, is thereby composed of common components, facile to prepare, and exhibit room temperature fluidity. The permanent porosity are manifested by the high-pressure isotherm for CH4 and modeling studies. With evidence from time-dependent amino acid uptake, competitive extraction studies and molecular dynamic simulations, HD/THTP_Cl exhibit better selectivity towards L-Trp than existing solid state sorbents, and we attribute it to not only the intrinsic porosity of HD but also the host-guest interactions between HD and L-Trp. Specifically, each HD unit is binded with nearly 5 L-Trp molecules, which is higher than the L-Trp occupation in the structure unit of other benchmark metal-organic frameworks.